Headaches

Ok so for the past week, I've been having headaches... Like a lot of them. They always start during school around 5th hour and they last until I fall asleep at night. That's like 11-12 hour headaches and they're killing me. At first I thought it was because I wasn't eating enough or because I was dehydrated, but then I started eating more and drinking like 4 bottles of water a day and they're still going on. Can y'all help me out?